The present invention concerns a process of warm compacting steel powder compositions as well as the compacted and sintered bodies obtained thereof. Specifically the invention concerns warm compacting of stainless steel powder compositions.
Since the start of the industrial use of powder metallurgical processes i.e. the pressing and sintering of metal powders, great efforts have been made in order to enhance the mechanical properties of P/M-components and to improve the tolerances of the finished parts in order to expand the market and achieve the lowest total cost.
Recently much attention has been paid to warm compaction as a promising way of improving the properties of P/M components. The warm compaction process gives the opportunity to increase the density level, i.e. decrease the porosity level in finished parts. The warm compaction process is applicable to most powder/material systems. Normally the warm compaction process leads to higher strength and better dimensional tolerances. A possibility of green machining, i.e. machining in the xe2x80x9cas-pressedxe2x80x9d state, is also obtained by this process.
Warm compaction is considered to be defined as compaction of a particulate material mostly consisting of metal powder above approximately 100xc2x0 C. up to approximately 150xc2x0 C. according to the currently available powder technologies such as Densmix, Ancorbond or Flow-Met.
A detailed description of the warm compaction process is described in e.g. a paper presented at PM TEC 96 World Congress, Washington, June 1996, which is hereby incorporated by reference. Specific types of lubricants used for warm compaction of iron powders are disclosed in e.g. the U.S. Pat. Nos. 5,154,881 and 5,744,433.
In the case of stainless steel powders it has now been found, however, the general advantages with warm compaction have been insignificant as only minor differences in e.g. density and green strength have been demonstrated. Additional and major problems encountered when warm compacting stainless steel powders are the high ejection forces and the high internal friction during compaction.
It has now unexpectedly been found that these problems can be eliminated and that a substantial increase in green strength and density can be obtained provided that the stainless steel powder is distinguished by very low oxygen, low silicon and carbon contents. More specifically the oxygen content should be below 0.20, preferably below 0.15 and most preferably below 0.10 and the carbon content should be lower than 0.03, preferably below 0.02 and most preferably below 0.01% by weight. The experiments also indicate that the silicon content is an important factor and that a silicon content should be low, preferably below about 0.5%, more preferably below 0.3% and most preferably below 0.2% by weight, in order to eliminate the problems encountered when stainless steel powders are warm compacted. Another finding is that the warm compaction of this stainless steel powder is most effective at high compaction pressures, i.e. that the density differences of the warm compacted and cold compacted bodies of this powder increase with increasing compaction pressures, which is quite contrary to the performance of standard iron or steel powders.
Preferably the powders subjected to warm compaction are pre-alloyed water atomised powders which include, by percent of weight, 10-30% of chromium, 0-5% of molybdenum, 0-15% of nickel, 0-0.5% of silicon, 0-1.5% of manganese, 0-2% of niobium, 0-2% of titanium, 0-2% of vanadium, 0-5% of Fe3P, 0-0.4% graphite and at most 0.3% of inevitable impurities and most preferably 10-20% of chromium, 0-3% of molybdenum, 0.1-0.3% of silicon, 0.1-0.4% of manganese, 0-0.5% of niobium, 0-0.5% of titanium, 0-0.5% of vanadium, 0-0.2% of graphite and essentially no nickel or alternatively 7-10% of nickel, the balance being iron and unavoidable impurities. The preparation of such powders is disclosed in the PCT patent application SE98/01189, which is hereby incorporated by reference.
The lubricant may be of any type as long as it is compatible with the warm compaction process. More specifically the lubricant should be a high temperature lubricant selected from the group consisting metal stearates, such as lithium stearates, paraffins, waxes, natural and synthetic fat derivatives. Also polyamides of the type disclosed in e.g. the U.S. Pat. Nos. 5,154,881 and 5,744,433, which are referred to above and which are hereby incorporated by reference, can be used. The lubricant is normally used in amounts between 0.1 and 2.0% by weight of the total composition.
According to one embodiment the mixture including the iron powder and high temperature lubricant may also include a binding agent. This agent might e.g. be selected from cellulose esters. If present, the binding agent is normally used in an amount of 0.01-0.40% by weight of the composition.
Optionally, but not necessarily, the powder mixture including the lubricant and an optional binding agent is heated to a temperature of 80-150xc2x0 C., preferably 100-120xc2x0 C. The heated mixture is then compacted in a tool heated to 80-130xc2x0 C., preferably 100-120xc2x0 C.
The obtained green bodies are then sintered in the same way as the standard materials, i.e. at temperatures between 1100xc2x0 C. and 1300xc2x0 C., the most pronounced advantages being obtained when the sintering is performed between 1120 and 1170xc2x0 C. as in this temperature interval the warm compacted material will maintain significantly higher density compared with the standard material. Furthermore the sintering is preferably carried out in standard non oxidative atmosphere for periods between 15 and 90, preferably between 20 and 60 minutes. The high densities according to the invention are obtained without the need of recompacting, resintering and/or sintering in inert atmosphere or vacuum.